That was my fear too about cancer. The mRNA teaches the body to fabricate stuff and that is foreign. This is crazy conditioning. I mentioned to someone that it would be like teaching the body to give itself cancer to fight it back better, schizophrenically...
It isn't a foreign at all. You would not exist without mRNA. mRNA being developed by the body's cells is what produces proteins. That fabrication process is a key function of many cells. This is very basic biology.
You have also almost certainly been infected with numerous viruses over your lifetime. They inject their own mRNA into your cells which commands the cells to produce more viruses. That is how viruses reproduce - they do not reproduce on their own. You get infected because your own body turns into a virus making factory. You don't escape this reality by not taking the vaccine.